1. Field of the Invention
The invention relates to a method for measuring the concentration of at least one gas component in a measurement gas.
2. Description of the Related Art
In a conventional method described in EP 1 693 665 A1, a measurement gas with a gas component to be determined therein is guided through a measurement volume comprising a measurement cuvette or, for in-situ measurements, comprising a tube or another system part conveying gas. The measurement volume has two windows with an optical measurement path of predetermined length lying between them. A wavelength-tunable light source, for example, a diode laser, generates a light beam, having a wavelength that is tuned to an absorption line of the gas component to be determined and which is directed through the measurement volume onto a detector. The signal generated by the detector is dependent on the light absorption in the measurement volume. As a result, the concentration of the gas component can be calculated from the signal while taking into account the specific absorption coefficient of the gas component and the known measurement path length.
The lower the concentration of the gas component to be measured is, the greater is the effect of perturbing absorptions that are due to components of the ambient air in the regions of the light path outside the measurement volume, i.e., between the light source and the measurement volume as well as between the measurement volume and the detector. It is therefore known to flush these regions with a gas that exhibits no absorption in the wavelength range of the absorption line of the gas component to be measured. A flushing gas suitable for many cases is, for example, nitrogen. Continuous flushing with fresh flushing gas, however, is associated with relatively high operating costs.
From the aforementioned EP 1 693 665 A1 publication, it is furthermore known to separately detect perturbing gas components, which as constituents of the ambient air may possibly enter the flushing gas path through a leak, and thereby correct the measurement result. To this end, the light from the light source is split into two sub-beams, one of which is directed through the measurement gas volume onto the aforementioned detector and the other of which is directed onto a second detector through a cuvette, through which the flushing gas flows.
From U.S. Pat. No. 5,747,809, it is known to guide the flushing gas in a circuit through a filter comprising an absorber substance, in order to remove perturbing gas components, which as constituents of the ambient air may possibly enter the flushing gas circuit through a leak. Since, depending on the effectiveness of the filter, the perturbing gas components may possibly be removed only incompletely, here again perturbing gas components entering the flushing gas are separately detected in order thereby to correct the measurement result. To this end, similarly as in the case of the EP 1 693 665 A1 publication, the light from the light source is split into two sub-beams, one of which is directed through the measurement gas volume onto the detector and the other of which is directed onto a second detector through a cuvette, through which the flushing gas flows.
From EP 2 000 792 A1, it is known to arrange a reference volume in series with the measurement volume in the light path. The reference volume contains the measurement gas component, for example oxygen, with a particular isotope, for example 18O2, in a particular 16 O:18O2 abundance ratio that is higher than the known natural abundance ratio of these isotopes in the gas component in the measurement volume. The absorption lines of the two isotopes 16O2 and 18O2 are scanned with the light. The concentration of the gas component in the measurement volume is calculated from the ratio of the detector signals at the peaks of the absorption lines based on Lambert's law and while taking into account the known isotope abundance ratios. For wavelength stabilization of the tunable light source, the wavelength may be locked to the absorption line of the 18O2 isotope. The problematic perturbing absorptions due to components of the ambient air in the regions of the light path outside the measurement volume and reference volume is not part of the subject-matter of EP 2 000 792 A1 and is not mentioned there.